A helicopter belonging to an air force unit stationed in the capital city of Hanoi crashed outside a rural village during a routine training exercise on Monday, leaving 18 dead and three injured.

The accident happened at around 7:50 a.m. when a Russian-made MI-171 chopper from Regiment 916 fell out of the sky with 21 people on board, including three crew members, two trainers, and 16 soldiers, during a parachuting exercise.

Sixteen people on board were killed and five others seriously burned. The injured have been treated at the National Institute for Burn Victims.

Two more soldiers succumbed to their injuries later in the evening, bringing the total fatalities to 18. The other three injured are still being treated.

Lieutenant General Vo Van Tuan, Deputy Chief of the Army General Staff, said the crash was due to "technical problems" and had nothing to do with a “sabotage plot from outside forces” as some may have assumed, Tuan was quoted by news website VnExpress as saying.

The helicopter took off from Hoa Lac military airport in Hanoi’s Ba Vi District at 7:30 a.m. and lost contact with the base 16 minutes later.

The helicopter reportedly made landfall near Village 11 in Thach Hoa Commune, Thach That District, around 500 meters away from a market and 40 kilometers west of Hanoi.

The crash site is around three kilometers from the airbase.

Witnesses to the accident said they heard a huge sound from the engine and saw smoke and fire rising from the helicopter as it descended from the sky.

After hearing an explosion, debris began falling out of the sky, eyewitnesses said. One chunk of metal crushed the motorbike and chair placed out in front of a local home, they said.

Shortly thereafter, the helicopter plunged into the field and burst into flames.

Dozens of young men from Village 11 rushed to the crash site and pulled people out of the flaming wreckage. They decided to pull away from the site shortly before the wreckage exploded again.

Local police and military forces soon cordoned off the entrance to the site while search and rescue work was underway.

By 10:30 a.m. rescue teams and firefighters managed to bring the last of the dead and injured to a local hospital.

Local residents said the number of casualties would be higher if the pilot had not managed to fly the helicopter toward the field and the helicopter had crashed into the Hoa Lac market.

Hoang Long, a resident who lives near the crash site, told news website VnExpress that he was startled when he saw the helicopter flying so low in the sky at around 7:40 a.m.; Long said it seemed as if it was plunging headlong into the ground.

Just a few moments later, he saw smoke, heard an explosion and ran into the field.

“One pilot died on the spot, around 20 others were totally burned, some of them were still holding parachutes in their hands,” Long said.

An investigation into the crash remains underway.

In 2008, five Vietnamese military pilots were killed when their twin-engine light transport aircraft crashed on the outskirts of Hanoi, according to an AFP report.

The Soviet-designed Antonov An-26 turboprop crashed into a rice field south of the capital during a morning training flight, the report said.